1. Field of the Invention
The present invention relates to a transmission equipped with a transfer having a shifting mechanism and, more particularly, to a control system for controlling such a transmission.
2. Description of the Prior Art
In a conventional transmission comprising a main transmission gear which converts a constant input speed, namely, the engine speed, into output speeds, and a transfer gear capable of establishing a two-wheel drive mode or a four-wheel drive mode, or establishing a gear ratio through manual selective operation, changeover between the two-wheel drive mode and the four-wheel drive mode is achieved by a hydraulic clutch, the speed change mechanism of the transfer gear employs a planetary gearing, the sun gear of the planetary gearing is connected to the input shaft of the transfer gear, the carrier of the planetary gearing is connected to the output shaft of the transfer gear, the sun gear and the carrier are engaged or disengaged by a hydraulic clutch, the ring gear of the planetary gearing is fixed or released by a hydraulic brake, the direct drive ratio is provided by engaging engaging the hydraulic clutch and disengaging the hydraulic brake, the reduction ratio is provided by disengaging the hydraulic clutch and engaging the hydraulic brake, the hydraulic clutch and the hydraulic brake are controlled manually, and the hydraulic servomechanisms of the hydraulic clutch and the hydraulic brake are controlled by a manual shift valve which generates a hydraulic signal corresponding to the speed change ratio and a low-high changeover valve which is driven by a hydraulic pressure (governor pressure) corresponding to the running speed of the vehicle. A hydraulic pressure corresponding to the running speed is applied to one end of the low-high changeover spool of the low-high changeover valve for controlling the hydraulic servomechanisms of the hydraulic clutch and the hydraulic brake to inhibit the establishment of the reduction ratio when the shift lever is changed from the direct drive ratio position to the reduction ratio position while the vehicle is running at a high speed so that the over revolution of the engine is prevented and inappropriate speed reduction is prevented.
According to such a prior art, the oil pressure corresponding to the running speed, namely, the governor pressure, is produced by a governor valve, however, such a governor valve is able to produce only pressures corresponding to the running speeds and unable to produce a pressure correctly representing various other running conditions of the vehicle. To improve such a drawback of the conventional transmission, a method has been proposed to control the transmission according to predetermined speed change characteristics. However, means for detecting the mode of the transfer gear has not been proposed.